This invention relates generally to a full frame copying device and, more particularly to an improved optical system adapted to provide a uniform irradiance at an image plane through a multi-magnification range.
In electrophotographic copiers, the areas of a charged photoconductive surface, which are irradiated by a light image, are dicharged, the degree of discharge dependent upon the intensity (irradiance) of the impinging light rays. It is, therefore, desirable that the light ray intensity vary only due to the reflectance characteristics of the original document being copied rather than due to changes introduced by the imaging components. Stated in another manner, the optimum system would be providing uniform photoconductor irradiance given a uniformly reflecting document.
Of the factors affecting relative irradiance at an imaging plane, the most significant is the cosine (cos) variation wherein the irradiance at an image plane is proportional to the cos.sup.4 of the angle between the lens axis and the field beam. Thus, even if an object plane is uniformly illuminated, photoconductor irradiance decreases as radial distance from the system optical axis increases. Various approaches have been devised to compensate for this effect. Typically, in slit-scanning systems, a sheet of opaque material having a butterfly slit formed thereon is employed with the irradiance profile. Other slit-scanning systems utilize a variable density filter in the optical path whose transmissiveness varies inversely to the cos.sup.4. Such a device is described in IBM Technical Disclosure, Vol. 14, No. 11 (April 1972). In addition to the cos.sup.4 variation, other circularly symmetrical non-uniformities such as lens exit pupil distortion, lens transmission and vignetting affect the relative irradiance at the imaging plane.
In full frame imaging systems wherein an entire document is typically illuminated by flash illumination, efforts to compensate for cos.sup.4 light falloff have emphasized locating light sources in such a way that the document edges are illuminated to a greater degree than central areas. Two such systems are disclosed in U.S. Pat. Nos. 3,669,538 (Fowler) and 3,777,135 (Rees). If these systems provide a magnification (generally reduction) option, a variation of the field angles (hence, cos.sup.4 variation) occurs with magnification changes, and it has not been possible to maintain the desired image plane illumination through all the reduction positions. This problem is further accentuated in system in which the document to be copies is edge or corner-registered due to the assymetrical aspect of the field.